The present invention relates to the display of images on image displays with different luminance rise and fall response times, such as liquid crystal displays, in particular to the display of TV pictures and/or data information on a video display system equipped with a liquid crystal display device.
The display of video images on display devices such as a Cathode Ray Tube (CRT) or a Liquid Crystal Display (LCD) is a known art. Image displays equipped with such CRT or LCD display devices are capable of displaying on a display screen images consisting of a number of picture elements (or pixels) which are refreshed at a refresh rate generally above 25 Hz. These images may be monochromatic, multicolour or full-colour. Common standards are in use to display the images as a succession of frames.
The light of the successive frames which are displayed on the display screen of such a CRT or LCD display device is integrated by the human eye. If the number of displayed frames per secondxe2x80x94further called the frame ratexe2x80x94is sufficiently high, an illusion of the images being displayed in a continuous way, and therefore an illusion of motion, can be created.
The way images are formed on the display screen of a CRT display device is fundamentally different from the way images are formed on the display screen of a LCD display device.
On a CRT display device, the luminance of a picture element is produced by an area of a phosphor layer in the display screen when said area is hit by a writing electron beam.
On a LCD display device, the luminance of a picture element is determined by the light transmittance state of one or more liquid crystal elements in the display screen of the LCD display device at the location of the picture element, whereby the light itself originates from ambient light or a light source.
For a faithful reproduction of moving images or moving parts of an image, the luminance response of the display device being used is of utmost importance.
The luminance responses and the luminance response times of display screens are known to be very different for CRT and LCD display devices. The luminance response time, being the time needed to reach the correct luminance on the display screen in response to an immediate change in a corresponding drive signal is shorter than a frame period for a CRT display device but up to several frame periods for a typical LCD display device according to the state of the art.
For LCD display devices, the luminance responses and luminance response times are also known to be different for a darker-to-brighter luminance transition as compared to the responses and response times for a similar brighter-to-darker luminance transition. Furthermore, the luminance responses and luminance response times are temperature dependent, drive voltage range dependent and, due to production tolerances, unequal over the LCD screen area (location dependent).
Various solutions are known for changing luminance response times with LCD display devices. They however have the aim to shorten the overall luminance response times, not to make the luminance rise and fall times equal. EP 0 487 140 discloses a method for speeding up display response times by doubling the display frame rate. The luminance rise and fall times remain different. EP 0 553 865 describes luminance flicker phenomena related to luminance response, but these phenomena are not due to the difference between luminance rise and fall times, but rather to how image lines are written.
There exist a number of images, further referred to as specific images, which when moved over a display screen with different luminance rise and fall times, give rise to visible and measurable artefacts in the displayed image, even when the luminance responses are shortened.
It is characteristic of such specific images that they contain a number of isolated or clustered picture points, which are in high contrast to their surroundings in the image.
The artefacts are due to the difference between luminance rise and fall times, which is typical for an LCD display device. This causes the luminance fall (or rise) of a white spot at a first location to be different from the simultaneous luminance rise (or fall) of a white spot at a second location, when the white spot is moved from the first to the second location. The total luminance integrated over the screen area immediately before, during and after the movement of the white point is not constant. The integrated luminance shows a xe2x80x98luminance jumpxe2x80x99.
In practice, the artefacts will only be visible when more picture elements change luminance at the same time within the observation field of the viewer.
In practice, various different artefacts may appear dependent on various parameters such as the difference between luminance rise and fall times, the frame rate of the displayed image, the video signal levels, the speed with which the image is moved over the screen, the image content.
The visible artefacts cause the quality of the displayed image to range from being inferior to unacceptable. The known solutions of increasing the frame rate do not fundamentally solve the problems but only make them in the best case less perceptible.
It is the aim of this present invention to remove luminance jumps and visible artefacts resulting from said luminance jumps in a displayed image during and immediately after the movement of the image, the luminance jumps and the artefacts caused by a difference in luminance rise and fall times of the display screen on which the image is displayed.
This is obtained by a method for converting a first video signal into a second video signal, the second video signal being intended for being displayed on a display device with different luminance rise and fall times, which comprises a display screen, and which operates at a frame period. The conversion is so that the second video signal causes the luminance time response of a picture element of the image to a change of the first video signal from a first amplitude value to a second amplitude value to be substantially equal in shape and amplitude but reversed (i.e., inverted) in slope compared to the luminance time response of the same or another picture element of the image to a change of the first video signal from the second amplitude value to the first amplitude value. The luminance time responses can be made substantially equal to xe2x80x98predefined luminance time responsesxe2x80x99.
The luminance time responses can be made substantially equal in amplitude and not slower than the luminance response of the same or another picture element which would be caused by the first video signal if this were displayed without conversion. The choice of the same or another picture element can be the same picture element itself, a reference picture element from a selected group of picture elements (e.g. a window) to which the same picture element belongs, any picture element which can be displayed on the display screen of the display device. The chosen same or another picture element can be that picture element of all picture elements which are aimed to be displayed of which the luminance response is the slowest. The conversion permits the compensation of the unevenness of the luminance rise and fall times over the surface of the display screen, as well as the compensation of the temperature dependency of the luminance rise and fall times.
According to a preferred embodiment, the conversion is such that the second video signal is built up in real time in consecutive steps during corresponding consecutive correction periods.
For the determination of a next step, one or more of the following parameters may be taken into account at the start of a correction period:
the present luminance of the picture element as predicted at the instant of the previous correction period,
the present amplitude of the first video signal,
the physical location of the picture element on the display screen,
the present temperature at the location of the picture element.
Preferably, a correction period is equal to a multiple of the frame period of the second video signal.
Preferably, the frame rate of the second video signal is a multiple of the frame rate of the first video signal.
According to an embodiment of the present invention, the conversion of the first video signal into the second video signal is so that the faster luminance response of a picture element to a change of the first video signal is slowed down in order to match the luminance response in time and amplitude to the known slower luminance response of the same or another picture element for the opposite change of the first video signal.
According to another embodiment of the present invention, the conversion of the first video signal to the second video signal is so that the slower luminance response of a picture element to a change of the first video signal is accelerated in order to match the luminance response in time and amplitude to the known faster luminance response of the same or another picture element for the opposite change of the first video signal.
According to another embodiment of the present invention, the conversion of the first video signal to the second video signal is so that the second video signal causes the luminance time response of a picture element to a change of the first video signal from a first amplitude value to a second amplitude value to be substantially equal in shape and amplitude but inverted in slope compared to the luminance time response of the same or another picture element for a change of the first video signal from the second amplitude value to the said first amplitude level, the luminance responses being equal to predefined luminance responses.
Furthermore, an apparatus is disclosed and claimed for carrying out a method as described herein.